PROJECT SUMMARY Obesity presents a major health concern as it is often a predisposing factor for multiple diseases, most notably heart disease and diabetes. There are two types of adipose tissue: white adipose tissue (WAT) which stores triglycerides and brown adipose tissue (BAT) which is an important site of non-shivering thermogenesis. Cells in the WAT depot can increase a population of cells, called ?beige? or ?brite? adipocytes, that have elevated levels of mitochondria, express uncoupling protein-1 (UCP1), and are more thermogenic. Increasing energy expenditure by increasing the amounts of beige and BAT is a potential anti-obesity therapeutic. Natriuretic peptides (NP) and nitric oxide (NO) can increase UCP1 expression and thermogenesis in adipocytes by activating cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG) signaling. Phosphodiesterase (PDE) enzymes break the phosphodiester bond in cyclic nucleotides rendering them inactive. PDE 5, 6, and 9 are specific to cGMP and, of these, PDE9 has the highest affinity for and is the most selective for cGMP. Inhibition of PDE9 will be investigated as a potential anti-obesity therapeutic target. Mice with a targeted gene deletion of Pde9 and PDE9 specific antagonists will be investigated for their potential to increase thermogenesis in adipocytes. The specific aims of this project are to: (1) Test the hypothesis that the cGMP-specific phosphodiesterase 9 modulates adipocyte metabolism and 'thermogenic' gene expression. In these studies the ability of the cGMP-specific phosphodiesterase 9 (PDE9) to regulate adipocyte metabolism will be investigated in vitro. The response to ANP for cGMP generation, glucose and fatty acid uptake, and energy utilization will be determined. Mitochondrial biogenesis, UCP1 expression, and cellular respiration will be measured as functional readouts of browning of adipose tissue in vitro. (2) Establish the impact of phosphodiesterase 9 on obesity and the diabetic/insulin resistant phenotype using mouse models. The role of PDE9 in adipose tissue will be investigated in vivo. Diet-induced and genetic models of obesity will be examined to test the hypothesis that pharmacological blockade of PDE9 or Pde9 gene knockout in mice will diminish weight gain, reduce insulin resistance, and improve glucose homeostasis. Studies will ascertain whether changes in the metabolic phenotype of these mice are due to thermogenesis via increased brown adipose tissue activity and/or expansion of UCP1-expressing cells in white adipose depots.